1. FIELD OF THE INVENTION
This invention relates to an improved process for removing carbon dioxide containing acidic gases from normally gaseous mixtures containing them and more particularly relates to a process of accomplishing substantial removal of these acidic gases from normally gaseous mixtures by contacting the normally gaseous mixtures with a solution containing a sterically hindered amine, a tertiary amino alcohol and a solvent for said amines.
2. DESCRIPTION OF THE PRIOR ART
It is well known in the art to treat gases and liquids, such as mixtures containing acidic gases including CO.sub.2, H.sub.2 S, SO.sub.2, SO.sub.3, CS.sub.2, HCN, COS and oxygen and sulfur derivatives of C.sub.1 to C.sub.4 hydrocarbons with amine solutions to remove these acidic gases. The amine usually contacts the acidic gases and the liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution contacting the acidic fluid countercurrently.
The acid gas scrubbing processes known in the art can be generally broken into three (3) categories.
The first category is generally referred to as the aqueous amine process where relatively concentrated amine solutions are employed during the absorption. This type of process is often utilized in the manufacture of ammonia where nearly complete removal of the acid gas, such as CO.sub.2, is required. It is also used in those instances where an acid gas, such as CO.sub.2, occurs with other acid gases or where the partial pressure of the CO.sub.2 and other gases are low.
A second category is generally referred to as the aqueous base scrubbing process or "hot pot" process. In this type of process a small level of an amine is included as an activator for the aqueous based used in the scrubbing solution. This type of process is generally used where bulk removal of an acid gas, such as CO.sub.2, is required. This process also applies to situations where the CO.sub.2 and feed gas pressures are high. In such processes, useful results are achieved using aqueous potassium carbonate solutions and an amine activator.
A third category is generally referred to as the nonaqueous solvent process. In this process, water is a minor constituent of the scrubbing solution and the amine is dissolved in the liquid phase containing the solvent. In this process, up to 50% of the amine is dissolved in the liquid phase. This type of process is utilized for specialized applications where the partial pressure of CO.sub.2 is extremely high and/or where many acid gases are present, e.g., COS, CH.sub.3 SH and CS.sub.2.
The present invention pertains to an improved process for practicing the third category of the acid gas scrubbing processes described above, namely, the nonaqueous solvent process where up to 50% or more of the same absorbent is dissolved in the liquid phase containing a solvent for the amine.
Many industrial processes for removal of carbon dioxide containing acidic gases use regenerable solutions of amines which are continuously circulated between an absorption zone where the acidic gases, e.g., carbon dioxide, are absorbed and a regeneration zone where the amine containing absorption solution which is saturated with the acidic components is desorbed usually by steam stripping. The capital cost of these acid gas scrubbing processes is generally controlled by the size of the absorption and regeneration towers, the size of the reboilers for generating stripping steam, and the size of the condensers which condense spent stripping steam so that condensate may be returned to the system to maintain proper water balance.
The cost of operating such scrubbing plants is generally related to the amount of heat required for the removal of a given amount of acid gas, e.g., thermal efficiency, sometimes expressed as cubic feet of acid gas removed per pound of steam consumed. Means for reducing the costs in operating these industrial processes have focused on the use of absorbing systems or combinations of chemical absorbents which will operate more efficiently and effectively in acid gas scrubbing processes using existing equipment.
There are a number of patents which describe improvements to improve the efficiency of the above-described processes for removing acidic gases from gaseous mixtures. Some of these improvements are described below.
U.S. Pat. No. 2,360,861 teaches the use of cyclic tetramethylene sulfones for separating mixtures of organic compounds and U.S. Pat. Nos. 2,385,704 and 3,475,329 teach the extraction of SO.sub.2 with cyclotetramethylene sulfones.
U.S. Pat. No. 3,039,251 to Kamlet teaches the use of certain sulfones, such as cyclotetramethylene sulfone (i.e., sulfolane) and the homologues thereof, alone or in combination with various alkanolamines for removal of hydrogen sulfide, mercaptans and/or carbon dioxide from normally gaseous mixtures. According to this patent, one is enabled to purify and dehydrate natural and synthetic gases, such as natural gas, synthesis gas, producer gas, coke oven gas, etc. The amines disclosed in this patent include monoethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine and alpha-aminopropionic acid.
A number of improvements and variations on the process taught by U.S. Pat. No. 3,039,251 have appeared in the patent literature. These improvements generally involve the use of specific amines or classes of amines, addition of other additives, such as iodine or water, and the use of other solvents, such as amides in place of the sulfones, etc. Examples of the patents which disclose some of these improvements are as follows:
______________________________________ U.S. Pat. Nos. 3,161,461 3,642,431 3,347,621 3,653,809 3,352,631 3,656,887 3,363,989 3,656,905 3,377,138 3,658,462 3,463,603 3,681,015 3,502,428 3,716,620 3,532,467 3,719,749 3,551,102 3,764,665 3,551,106 3,767,766 3,553,936 3,777,010 3,565,573 3,801,708 3,577,221 3,843,512 3,618,331 3,928,548 3,630,666 3,965,244 3,632,519 3,965,253 British Patent Specification Numbers 957,260 1,131,989 972,160 1,153,789 997,169 1,158,976 1,058,304 1,238,696 1,118,687 Canadian Patent Numbers 951,494 729,090 Dutch Patent Specification Numbers 67/06653 73/12490 73/12491 German Offenlegungschrift Numbers 1,542,415 2,422,581 2,433,078 ______________________________________
None of these patents or patent publications disclose, teach or suggest the use of sterically hindered amines and a tertiary amino alcohol with a solvent such as sulfolane or the unexpected benefits of the instant invention described herein. British Pat. Specification Nos. 972,140, 1,058,304 and 1,238,696 and U.S. Pat. No. 3,716,620, however, are of particular interest with respect to the instant invention and, accordingly, are discussed in more detail hereinafter.
British Pat. Specification No. 972,140 (which generally corresponds to U.S. Pat. No. 3,161,461) discloses a process for removing acid gases from gaseous streams by contacting the gaseous streams with a composition comprising organic solvent, such as sulfolane, and at least one amine having a weakly basic character in the range of pK.sub.b at 25.degree. C. of 3-14, e.g., diethanolamine.
British Pat. Specification No. 1,058,304 describes a process for removing acid gases from gaseous streams by contacting the gaseous streams with an aqueous solution comprising sulfolane and a secondary alkanolamine or morpholine or derivatives of morpholine, e.g., 2,6-dimethylmorpholine, 2,6-dimethylmorpholine, 2,3,5,6-tetraethylmorpholine, 2-methylmorpholine, 2-ethylmorpholine and 2-methyl-6-ethylmorpholine. The patent specification does not teach the concept of improved working capacity as defined hereinafter or the use of sterically hindered primary alkanolamines, and more importantly the patent specification does not teach the use of a sterically hindered amine in combination with a tertiary amino alcohol and a solvent as provided by the instant invention.
British Pat. Specification No. 1,238,696 discloses a process for removing acid gases from gaseous streams by contacting the gaseous streams with a composition comprising an organic solvent and an alkanolamine such as cyclohexylamino ethanol. Sterically hindered amines as defined herein are not specifically taught in this patent specification.
U.S. Pat. No. 3,716,620 discloses a process for removing acid gases from gaseous streams by contacting the gaseous streams with a composition comprising a solution of iodine in an organic solvent which also contains an amine. Among the solvents disclosed, there are included the sulfones, e.g., sulfolane. Among the amines, there are disclosed alkanolamines with primary, secondary or tertiary amino groups having 2 to 12 carbon atoms and 1 to 3 amine groups, e.g., mono, di- and triethanolamines, N-methyl diethanolamine, N-cyclohexyldipropanolamine and diisopropanolamine.
Prior art workers have taught that sterically hindered amines would have low rates of combination with CO.sub.2 and apparently concluded, although other explanations are possible, that such sterically hindered amines would be inefficient in CO.sub.2 scrubbing processes. For example, Sharma, M. M., Trans. Faraday Soc., 61, 681-8 (1965) described the kinetics of reaction between CO.sub.2 and COS with 38 amines, some of which are sterically hindered amines. Other researchers have attributed relatively poor absorption rates of CO.sub.2 by amines to steric hindrance. See, for example, J. L. Frahn and J. A. Mills, Aust. J. Chem., 17, 256-73, 263 (1964) and M. B. Jensen, Acta Chemica Scandinavica, 11, 499-505 (1957).
Shrier and Danckwerts, Ind. Eng. Chem. Fundamentals, 8, 415 (1969) discussed the use of amines as promoters for aqueous carbon dioxide absorption solutions. However, these researchers only ran initial absorption rate experiments and did not recognize the unique capacity advantages obtained by using sterically hindered amines in an acid gas scrubbing process. Also of interest is Danckwerts and Sharma, The Chemical Engineer, Oct. 1966, pp 244-280.
U.S. Pat. No. 2,176,441 to Ulrich et al. teaches the use of amino acids having a primary, secondary or tertiary amino group and at least two nitrogen atoms to remove acidic gases. The patentees provide various general formulae for the amino acids taught to be useful in the acid gas scrubbing process. While certain "sterically hindered amines" can be derived by proper choice of substituent groups in the general formulae there is no teaching that these amines will achieve any unexpected results, such as improved regeneration rates coupled with high rates of absorption.
There are a number of patents which disclose the use of various amines as "activators" in an alkaline scrubbing solution wherein the primary absorbent is an alkaline salt such as potassium carbonate. Some of these processes are described in U.S. Pat. Nos. 2,718,454, 3,144,301, 3,637,345, 3,793,434, 3,848,057, 3,856,921, 3,563,695, 3,563,696 and 3,642,430, as well as some other patents such as Belgian Pat. No. 767,105; British Pat. Nos. 1,063,517, 1,218,083 and 1,305,718.
In the prior art processes discussed above, it is apparent that the efficiency of processes employing absorbing solutions is generally limited by the relatively slow rate of transfer of molecules of the acid gas from the gas phase to the liquid phase as well as in the regeneration of the absorbing solution. Many of the above-described prior art processes deal with means to render the acid gas scrubbing process more efficient.
In copending U.S. application Ser. No. 590,427, filed June 26, 1975, the disclosure of which is incorporated herein by reference, there is disclosed and claimed sterically hindered amine compositions useful for scrubbing acid gases. These sterically hindered amines unexpectedly improve the efficiency, effectiveness and working capacity of the acid gas scrubbing processes in all three of the abovementioned process categories. It was postulated in U.S. Ser. No. 590,427 that the increase in cyclic capacity observed with the sterically hindered amines is due to the instability of their carbamates. In that respect, sterically hindered amines are similar to tertiary amines. Tertiary amines are not used on a commercial scale for carbon dioxide containing acid gas scrubbing due to their low rates of absorption and desorption.
It has been observed that tertiary amino alcohols can replace a portion of the sterically hindered amines in the amine-solvent process disclosed in U.S. Ser. No. 590,427, and the advantages of improved cyclic capacity are still enjoyed. This advantage provides a reduction in the total amine cost owing to the lower cost of the tertiary amines compared to the sterically hindered amines.
It has also been discovered that the combination of tertiary amino alcohol and the sterically hindered amines provides a lower heat of reaction than for the amine-solvent solutions containing the sterically hindered amines alone.